Comm Synchronization En

8/9/2019 Comm Synchronization En

1/45

Synchronization Shanghai Jiao Tong University


2/45

Synchronization

Synchronization is one of the most critical functions of a communicationsystem with coherent receiver. To some extent, it is the basis of asynchronous communication system.

• Carrier synchronization

• Symbol/Bit synchronization

• Frame synchronization

2


3/45

Synchronization

Carrier synchronization ( Receiver needs estimate and com ensate for fre!uency

and hase differences between a received si"nal#s carrierwave and the receiver#s local oscillator for the ur ose of

coherent demodulation, no matter it is analo" or di"italcommunication systems

3


4/45

Frame synchronization $n frame%based di"ital systems, receiver also needs to estimate the

startin"/sto in" time of a data frame . The rocess of extractin" such acloc& si"nal is called frame synchronization.

Symbol/bit synchronization ( /$n di"ital systems, the out ut of the receivin" filter 'i.e. matched filter(must be sam led at the symbol rate and at the recise sam lin" timeinstants . )ence, we re!uire a cloc& si"nal. The rocess of extractin"such a cloc& si"nal at the receiver is called symbol/bit synchronization.

Synchronization

4


5/45

*++ is often used in carrier syn. and symbol syn. $t is a closed%loo control system consistin" of

*hase detector '* (- "enerate the hase difference of v i't( and v o't(.olta"e%controlled oscillator ' C (- ad0ust the oscillator fre!uency based on

this hase difference to eliminate the hase difference. 1t steady state, theout ut fre!uency will be exactly the same with the in ut fre!uency.

+oo filter '+F(

Phase-locked Loop

5


6/45

Phase-locked Loop

(2'cos3('

(2'sin3('

4

4

t t vt v

t t vt v

oo

ii

∧

+=

+=

φ ω

φ ω

('sin5 (2'('sin35 (' t t t t v ed d d φ φ φ =−= ∧

+oo filter is also a +*F.

1 * contains a multi lier and a low ass filter. The out ut of * is-

('('(' t v p F t v d c =The out ut of the +F is 'where F' ( is the transfer function(

6


7/45

Phase-locked Loop

('5 (' t v

dt

t d cv=

∧

φ

The out ut of C can be a sinusoid or a eriodic im ulse train. The

differentiation of the out ut fre!uency are lar"ely ro ortional to the in utvolta"e.

$f F' (67 Then

('sin5 ('

t dt

t d eφ

φ =∧

This &ind of loo is called the first%order loo

7


8/45

Digital PLL

8

i"ital

*

i"ital +*

i"ital C

*++$n ut

out ut8%9 *++


9/45

Phase-locked Loop

$n a coherence system, a *++ is used for-7. *++ can trac& the in ut fre!uency and "enerate the out ut si"nal with

small hase difference.:. *++ has the character of narrowband filterin" which can eliminate the

noise introduced by modulation and reduce the additive noise.

;.


10/45

Carrier Synchronization

To extract the carrier

7. *ilot%tone insertion method Sendin" a carrier com onent at s ecific s ectral%line alon" with the si"nalcom onent. Since the inserted carrier com onent has hi"h fre!uency stability,it is called ilot ' (.

:. irect extraction method irectly extract the synchronization information from the receivedsi"nal com onent.

10


11/45

Pilot-tone insertion method

Modulator Bandpassilter

!dd s"t#

$"t#

π %&phase shi t

-asin" ω c t#

cos" ω c t#

7. *ilot%tone insertion methodinsert ilot to the modulated si"nal Thinking'hy () )

shi t

The ilot si"nal is "enerated by shift the carrier by 84 4 and decrease by severaldB, then add to the modulated si"nal. 1ssume the modulated si"nal has 4 Ccom onent, then the ilot is

11


12/45

*" #

$)

S" #

)

cc - $ c + $


The receiver uses a narrowband filter with central fre!uency f c to extract the

ilot and then the carrier can be "enerated by sim lyshiftin" 84 4.

12


13/45


SB, SSB and *S5 are all ca able of ilot%tone insertion method. SB canalso a ly ilot%tone insertion method but with certain modification.

13


14/45

Narrowband Filter

The drawbac& of narrowband filter-• The ass band is not narrow enou"h• f c is fixed, cannot tolerate any fre!uency drift with res ect to the centralfre!uency

•Can be re laced by *++

*ilot%tone insertion method is suitable for SB, SSB, S and :*S5

14


15/45

Direct extraction method

&, irect extraction method

#, $f the s ectrum of the received si"nal already contains carriercom onent, then the carrier com onent can be extracted sim ly by anarrowband filter or a *++.

, $f the modulated si"nal su resses the carrier com onent, then thecarrier com onent may be extracted by erformin" nonlineartransformation or usin" a *++ with s ecific desi"n.

15


16/45

.onlinear-trans or/ation-0ased /ethod

7. S1uare trans or/ation

: = ,mcmc

16


17/45


18/45

.onlinear-trans or/ation-0ased /ethod

2 S!uare *++

18


19/45

3n-phase orthogonal loop4Costas Loop

&, $n% hase ortho"onal loo >Costas +oo

Contains in-phase 0ranch andorthogonal 0ranch, !ll parts e$ceptL5 and 6C7 are si/ilar 8ith a 9phasedetector:,

19

77 e ee eenr == s sma: ::77cos::7;?e1 ed rt tt rtrt tvt =→⋅→=o"erranc


20/45

3n-phase orthogonal loop4Costas Loop..20


21/45

Per or/ance

;, *erformance of carrier synchronizationtechni!ue

7( *hase error- steady%state hase error, random hase error

:( Synchronization build time and hold time

21


22/45

Symbol Synchronization

In a digital communication system, the output of the receivingfilter must be sampled periodically at the symbol rate and at theprecise sampling time instance!o perform this periodic sampling, we need a cloc" signal at thereceiver!he process of extracting such a cloc" signal is called symbol

synchroni#ation or timing recovery$ne method is for the transmitter to simultaneously transmit thecloc" fre%uency along with the information signal !he receive cansimply employ a narrowband filter or PLL to extract it !hismethod re%uires extra power and bandwidth and hence, butfre%uently used in telephone transmission systems&nother method is to extract the cloc" signal from the receiveddata signal by using some "ind of non-linear transformation

22


23/45

'arly-Late (ate )ynchroni#ation*asic Idea+ exploit the symmetry properties of the output

signal of matched filter or correlator

Due to the symmetry, the values of thecorrelation function at the early samples

and the late samples are e%ual!hus, the proper sampling time is the midpoint

between and

23


24/45

24


25/45

.onlinear-trans or/ation-0ased/ethod7. @onlinear%transformation%based method

Some transformations can add synchronous si"nal with f67/T to theori"inal si"nal. For exam le, we can transform the si"nal to return%to%zerowaveform. 1fter narrowband filterin" and hase shiftin", we can "eneratethe cloc& si"nal used for synchronization.

25


26/45


27/45


28/45

Per or/ance

;. *erformance of symbol synchronization system

> *++

7(. *hase error

:(. Synchronization build time

;(. Synchronization hold time

?(. Synchronous bandwidth

28


29/45

Frame Synchronization

Aecall that carrier and symbol synchronization needs to estimatethe hase of synchronous si"nal which can be realized by usin" a*++.

Frame synchronization is to insert frame ali"nment si"nal'distinctive bit se!uence( and then detect the ali"nment symbol.

Besides addin" frame ali"nment bits, some code such as self%synchronizin" code can be synchronized without addin" extra bits.

)ere, we only focus on the first method >>insertin" frameali"nment si"nal.

29


30/45

Frame Synchronization

Start%sto methodBunched frame ali"nment si"nal

istributed frame ali"nment si"nal

30


31/45

Start-stop Method

7. Start%sto method $t is widely used in tele rinter. =ach symbol contains % data bits, a start bit and

a sto bit.

..31


32/45

Drawbacks:

7(. +ow transmission efficiency

:(. +ow timin" accuracy

Start-stop Method

32


33/45

Bunched frame ali"nment si"nal

&, Bunched frame ali"nment si"nal

This method inserts synchronous code at a articular lace ineach frame. The code should have a shar self%correlation function .The detector should be sim le to im lement.

Frame synchronization code-

Bar&er code

o timal synchronous code

seudo%random code.

33


34/45

,,,n xi−+= ar ercoesnoaer ocseuence.sroveaenar ercoew.n=*ar"er ode

34


35/45

n barker code: G G

; G G %

? G G G % G G % G

G G G % G

G G G % % G %

77 G G G % % % G % % G %

7; G G G G % % G G % G % G

*ar"er ode

35


36/45

. x *ar"er ode36


37/45

':( Bar&er code "enerator

>shift re"ister

=xam le when n6 , a bits shift re"ister. The initial state is a bar&er code.

*ar"er ode

37


38/45

";# Barker code detector

*ar"er ode

38

p:tt


39/45

upu :utput :wenenusaar ercoeeouuoesreser seecorwsenasncronousmuse.*ar"er ode

39


40/45

istributed frame ali"nment si"nal

;, istributed frame ali"nment si"nal

The synchronous code is distributed in the data si"nal. That means between each n bits, a synchronous bit is inserted.

esi"n criteria of synchronous code -

7. =asy to detect. For exam le- H77777777Ior I74747474I

:. =asy to se arate synchronous code from data code. For exam le- $nsome di"ital tele hone system all I4I stands for rin", so synchronous codecan only use H74747474I.

40


41/45

Per or/ance

Per or/ance o ra/e synchronization syste/

4Bunched frame ali"nment si"nal7. *robability of missin" synchronization PL

1ffected by noise, the detector may not be able to detect thesynchronous code. The robability of this situation is called robability ofmissin" synchronization * +.

1ssume the len"th of synchronous code is n bit error rate is *e. Thedetector will not be able to detect if more than m bit errors ha en, then-

41


42/45

&, *robability of false synchronization P5

Since data code can be arbitrary, it may be the same with synchronouscode. The robability of this situation is called robability of false synchronization* F.

* F e!uals to the robability of a earance of synchronous code in the data code.

a. $n a binary code, assume 4 and 7 a ears with the same robability. There are : n combinations of a n bit code.

b. 1ssume when there are more than m bit errors, the data code will also be detectedas synchronous code.

Per or/ance

...... xn x F ==m44mn xnn x==42


43/45

Per or/ance 43


44/45

. ,,7s . ,sees,eaveraeumeousnesbLF =::7:s r ueraea1arentsoterevousmetosoreese. sb st tt =>><Per or/ance

44


45/45

)uggested eading

!" $% &'()*+, , .//0 1, -./

hapter 0 1 of Fundamentals of CommunicationsSystems , Pearson Prentice 2all .//3, by Proa"is4 )alehi

Documents

Comm Synchronization En